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signal frequency produced from it was close to, or equal to, that of the wanted signal
then it is clear that a spurious output would result.
For example, at the sampling frequency of 1.0 MHz then half the sampling frequency
is 500 kHz. If a signal of 40 kHz accompanied by an interfering signal of 450 kHz
was then applied, the output of the ADC would include a sampled-data sinusoid of
40 kHz (the required signal) and, applying the above formula, an alias signal of
50 kHz (i.e. |500 kHz - 450 kHz|). If the signal frequency were now increased
towards 50 kHz then the output of the lock-in amplifier would increasingly be
affected by the presence of the alias signal and the accuracy of the measurement
would deteriorate.
To overcome this problem the signal is fed through the anti-aliasing filter which
restricts the signal bandwidth to an upper frequency of less than 200 kHz The filter is
a conventional elliptic-type, low-pass, stage, giving the lowest possible noise
bandwidth.
It should be noted that the dynamic range of a lock-in amplifier is normally so high
that practical anti-alias filters are not capable of completely removing the effect of a
full-scale alias. For instance, even if the filter gives 100 dB attenuation, an alias at
the input limit and at the reference frequency will give a one percent output error
when the dynamic reserve is set to 60 dB, or a ten percent error when the dynamic
reserve is set to 80 dB.
In a typical low-level signal recovery situation, many unwanted inputs need to be
dealt with and it is normal practice to make small adjustments to the reference
frequency until a clear point on the frequency spectrum is reached. In this context an
unwanted alias is treated as just another interfering signal and its frequency is
avoided when setting the reference frequency.
A buffered version of the analog signal just prior to transmission via the fiber link is
available at the signal monitor (SIG MON) connector on the rear panel of the
7124 RCU; it may be viewed on an oscilloscope to monitor the effect of the line
frequency rejection and anti-aliasing filters and signal-channel amplifiers.
3.3.06 Analog Fiber Links
The 7124 RCU is connected to the main console via four fiber optic links. Two of
these, the first for the main signal input from the RCU to the main console, and the
second carrying the oscillator signal from the main console to the RCU, carry purely
analog signals. The system includes the ability to handle variation in the optical
attenuation of the link by means of built-in calibration routines and additional circuit
functions not shown in figures 3-1 and 3-2.
3.3.07 Auxiliary Signal Input - Main Console
The front panel AUX connector on the main console provides an alternative input to
the main signal channel ADC. The input is a single-ended voltage mode input, and,
as is clear from the block diagrams it is of fixed full-scale sensitivity. There is also
no provision for a line frequency rejection filter, although there is an anti-aliasing
filter.
Chapter 3, TECHNICAL DESCRIPTION
3-7
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